Gel coats are used in marine and other applications to provide a smooth, attractive surface to the exterior of fiberglass-reinforced products and to protect laminates from the environment. A gel coat is a surface coating that usually contains pigments, resin, fillers, thixotropic agents, UV stabilizers, and promoters. When applied to a mold surface, it cures with structural layers and reproduces contours of the mold surface while sealing in layers of reinforcing fiber. Most gel coats are formulated using unsaturated polyester, vinyl ester, or epoxy resins. When high performance is needed (as in marine or shower/bath applications), unsaturated polyester resins made from isophthalic acid, maleic anhydride, and neopentyl glycol are often selected.
In a typical in-mold process, a gel coat is sprayed or brushed onto a mold surface and allowed to partially cure. A skin coat of resin is applied and cured, and then structural layers that contain fiber reinforcement are subsequently applied.
Although unsaturated polyester and vinyl ester gel coats are widely used, they tend to be relatively brittle. Because of their lack of toughness, the coatings can crack or chip. In fact, gel coat-related issues account for about half of recreational boat warranty claims.
Polyurethane or polyurea coatings are sometimes used as an alternative to a polyester gel coat. Urethanes offer the potential advantages of improved flexibility and toughness. However, urethanes need to be applied after manufacture. When urethanes are applied in-mold as gel coats, the unsaturated polyester-based structural layers do not adhere well enough to the gel coat. In some cases, it is possible to use a fiber tie coat to create a mechanical bond. For instance, an incompletely saturated spun-bonded polyester fabric can be placed into the uncured urethane coating and allowed to cure, followed by infusion with resin and lamination with structural layers. Here, the unsaturated fabric acts as a mechanical interface between the coating and structural layers. However, although this provides desirable adhesion, it is labor intensive, and in large parts, gaining access to place the tie-coat can be problematic.
Unsaturated polyester structural layers do not bond well to cured urethanes, and even post-applied urethane coatings can delaminate or peel, particularly under hydrodynamic conditions (as in a speedboat). No racer wants to cross the finish line in last place, but especially not while also towing an unsightly “bag” of seawater. Thus, despite the potential flexibility and toughness of urethanes, they have not displaced traditional unsaturated polyester gel coats.
“Hybrid” urethane/polyester systems were developed in the early 1980s at Amoco and were subsequently popularized elsewhere (see, e.g., U.S. Pat. Nos. 4,280,979; 4,822,849; 4,892,919; 5,153,261; 5,159,044; 5,296,544; 5,344,852). Most of these disclosures focus on foams or molded systems, with less emphasis on gel coats, although there is some use of hybrid systems for gel coating (see U.S. Pat. Appl. Publ. Nos. 2007/0049686 and 2008/0160307). The prototypical hybrid system has two components: an “A side,” which is a mixture of a polyisocyanate and methyl ethyl ketone peroxide (MEKP, a part of the catalyst used to cure the unsaturated polyester resin); and a “B side,” which includes a hydroxyl-terminated unsaturated polyester polyol, styrene, fillers, pigments, a glycol chain extender, a cobalt compound (the other half of the polyester curative), and a tin catalyst (urethane catalyst). When the A and B sides are combined, both polyurethane and polyester curing reactions occur. The hydroxyl-terminated unsaturated polyester polyol participates in both curing processes, as its hydroxyl groups react with the polyisocyanate and its carbon-carbon double bonds react with styrene and the radical curative.
Each of the hybrid systems discussed above requires the synthesis and use of a hydroxyl-terminated unsaturated polyester polyol, a material that is not normally used in either a conventional polyurethane (which uses saturated polyether or polyester polyols) or polyester system (which has unsaturation but not substantial hydroxyl end group content).
Hybrid systems are still available to a limited degree commercially, although in an evolved form. For instance, CCP Composites sells products under the Xycon® mark for use in pultrusion that are “modified thermosetting acrylic resins containing styrene monomer.” These are combined with an aromatic prepolymer to produce tough, heat and water-resistant polymers. Hydroxy functionality is frequently introduced by using hydroxyacrylate monomers (hydroxyethyl acrylate, hydroxyethyl methacrylate), which can be pricey. For some disclosures of urethane/acrylate systems, see U.S. Pat. No. 7,150,915 or U.S. Pat. Appl. Publ. No. 2007/0001343.
Co-cured systems of urethanes and polyesters have been described, frequently in the context of academic papers related to the study of properties of interpenetrating networks (IPNs). For just two examples, see X. Ramis et al., Polymer 42 (2001) 9469 or G. Y. Wang et al., Eur. Polym. J. 36 (2000) 735. As noted in the latter paper, a true IPN does not have chemical bonds between the networks, but a co-cured system involving commercial urethane and polyester systems would have some reaction of urethane —NCO groups with polyester —OH groups.
U.S. Pat. No. 5,952,436 describes a co-cured product made by reacting styrene, a polyisocyanate, and a polyetherester resin. The polyetherester resin is made by inserting maleic anhydride into C—O bonds of a polyether polyol.
U.S. Pat. No. 5,936,034 combines a traditional unsaturated polyester gel coat, though incompletely cured, and a co-cured “backer” layer made from an unsaturated polyester resin and an isocyanate-terminated prepolymer.
Despite the wealth of literature related to hybrid systems and IPNs, relatively little commercial success has been achieved with co-cured urethane and unsaturated polyester/vinyl ester resins. Most gel coats are still applied as in-mold coatings with unsaturated polyester resins, and most urethanes are applied as post-manufacture coatings. The issues of relatively poor adhesion between these traditional systems have yet to be completely resolved.
The industry would benefit from the availability of gel coatings that have improved toughness, flexibility, and chip- or crack-resistance compared with unsaturated polyester gel coats. Preferably, these benefits could be achieved without sacrificing good adhesion to structural layers or the convenience of using an in-mold coating process. Ideally, the gel coat could be formulated with commercially available materials, thereby overcoming the need to synthesize a special reactant, as is used in the hybrid systems discussed above. The industry would also value elastomeric coatings having improved adhesion to structural layers and a reduced tendency to delaminate from them compared with traditional urethane coatings. Ideally, the elastomeric coating could be used in an in-mold process. Finally, the industry would benefit from the ability to fabricate fiber-reinforced structural layers that can adhere well to coatings and to each other. Preferably, these layers could also have varying degrees of strain to failure, toughness, stiffness, or flexibility. The ability to “dial in” properties for each layer in a laminate would be valuable.